The present invention relates to methods and devices for completion of well bores and more particularly, to reverse circulation cementing of casing strings in well bores.
Conventional methods for completion of well bores typically involve cementing a casing string or multiple casing strings in a well bore. Cementing of a casing string is often accomplished by pumping a cement slurry down the inside of a tubing, a casing, and then back up the annular space around the casing. In this way, a cement slurry may be introduced into the annular space of the casing (e.g. the annular space between the casing to be cemented and the open hole or outer casing to which the casing is to be cemented).
Cementing in this fashion has several drawbacks. In particular, high pressures are required to “lift” the cement up into the annular space around the casing. These high delivery pressures may, in some cases, cause formation damage. Likewise, high delivery pressures can cause the undesirable effect of inadvertently “floating” the casing string. That is, exposing the bottom hole of the well bore to high delivery pressures can, in some cases, cause the casing string to “float” upward.
Another method of cementing casing, sometimes referred to as reverse circulation cementing, involves introducing the cement slurry directly from the surface into the annular space rather than introducing the cement slurry down the casing string itself. In particular, reverse circulation cementing avoids the higher pressures necessary to lift the cement slurry up the annulus. Other disadvantages of having to pump the cement slurry all the way down the casing string and then up the annulus are that it requires a much longer duration of time than reverse circulation cementing. This increased job time is disadvantageous because of the additional costs associated with a longer duration cementing job. Moreover, the additional time required often necessitates a longer set delay time, which may require additional set retarders or other chemicals to be added to the cement slurry.
Further, pumping a cement slurry all the way to the bottom hole of the well bore exposes the cement slurry to higher temperatures than would otherwise be necessary had the cement slurry been introduced directly from the surface to the annulus to be cemented. This exposure to higher temperatures at the bottom hole is undesirable, in part, because the higher temperatures may cause the cement to set prematurely or may cause the operator to modify the cement composition to be able to withstand the higher temperatures, which may result in a less desirable final cementing completion.
Thus, reverse circulation cementing has many advantages over conventional cementing. Nevertheless, reverse circulation cementing involves other challenges such as fluidic access to the annulus. Unfortunately, conventional methods for isolating the casing annulus either do not permit reverse circulation cementing or often involve complex and/or expensive equipment. In some cases, the equipment used for isolating the casing annulus for a reverse circulation cementing requires that the drilling rig remain at the well location for the duration of the cementing job. Requiring the drilling rig to stay at the well during a cementing operations is problematic in part because the drilling rig may not be used to drill subsequent wells during the cementing job and the cost of keeping the drilling rig on location is often quite high.